1. Field of the Invention
The present invention relates to a resetting circuit adapted for use in an optical information processing apparatus such as a facsimile apparatus, a video cassette recorder or the like.
2. Related Background Art
FIG. 1 is a circuit diagram of a photoelectric converting device including resetting circuits, as an example of the conventional resetting circuit; FIG. 2 is a circuit diagram of one of pixels S.sub.11, S.sub.12, . . . shown in FIG. 1; and FIG. 3 is a timing chart showing the function of said circuit.
In the conventional example shown in FIG. 1, pixels S.sub.11, S.sub.12 . . . S.sub.21, S.sub.22, . . . , each containing a bipolar transistor, are arranged two-dimensionally, wherein said pixels are connected, in respective rows, to the driving lines .phi..sub.V1, .phi..sub.V2, . . . of a vertical scanning circuit, while the output terminals of said pixels are connected, in respective columns, to output lines H.sub.1, H.sub.2, . . .
The output lines H.sub.1, H.sub.2, . . . are respectively connected to MOS transistors M.sub.11, M.sub.12, . . . constituting resetting switch means and MOS transistors M.sub.21, M.sub.22, . . . for signal transfer.
Other terminals of said resetting MOS transistors, not connected to said output lines, are connected to a resetting reference voltage source V.sub.VC, and the gates of said MOS transistors are controlled by clock signals .phi..sub.VC.
The output terminals of the signal transfer MOS transistors are respectively connected to accumulating capacitors C.sub.1, C.sub.2, . . . and further to MOS transistors M.sub.31, M.sub.32, . . . which are driven in succession by a horizontal scanning circuit. Other terminals of said MOS transistors M.sub.31, M.sub.32, . . . are connected to a common output line.
Each of the pixels S.sub.11, . . . is composed, as shown in FIG. 2, of a phototransistor Q.sub.1, a capacitor C.sub.OX connected between the base of said phototransistor Q.sub.1 and the line .phi..sub.V for controlling the base potential of the transistor Q.sub.1, a PMOS transistor M.sub.BR for resetting the base of the transistor Q.sub.1, a resetting variable voltage source V.sub.BR, and a collector power source V.sub.cc for the transistor Q.sub.1.
The transistor Q.sub.1 is connected, at the collector thereof, with V.sub.cc, also at the emitter with the output line H, and at the base with a terminal of the capacitor C.sub.OX and a terminal of the PMOS transistor M.sub.BR. The other terminal of the capacitor C.sub.OX and the gate of the PMOS transistor M.sub.BR are connected to the line .phi..sub.V, while the other terminal of the PMOS transistor M.sub.BR is connected to the variable voltage source V.sub.BR.
In the following there will be explained the functions of components with reference to a timing chart shown in FIG. 30 wherein the base potential and the emitter potential are those of the phototransistor Q.sub.1. V.sub.BRH means a high-level voltage of the variable voltage source V.sub.BR, generally a voltage of about +3 V, while V.sub.BRL is a low-level voltage of the variable voltage source V.sub.BR, which has to be lower than the threshold value of the PMOS transistor M.sub.BR for the functions of the circuit and is generally about .+-.0 V.
At first, at a timing t.sub.1, the PMOS transistor M.sub.BR is turned on to reset the base of the transistor Q.sub.1 to V.sub.BRH. The potential of the emitter of said transistor Q.sub.1, being in a floating state, is elevated following the base potential.
Then, at a timing t.sub.2, the transistors M.sub.11, M.sub.12, . . . are turned on by the block signal .phi..sub.VC, then the PMOS transistors M.sub.BR are turned off by an elevation of the potential .phi..sub.V, and the base potential is elevated by the capacitative coupling of the capacitors C.sub.OX to maintain the phototransistors of the pixels in a forward biased state, whereby the pixels are reset by the emitter-follower operation.
Then, at a timing t.sub.3, the potential .phi..sub.V is reduced to lower the base potential by the capacitative coupling of C.sub.OX, thereby maintaining the phototransistor in an inverse biased state. In this state, the voltage of the variable voltage source is reduced to V.sub.BRL, so that the resetting need not be repeated again by the turning-on of the PMOS transistor.
When light enters the base of the phototransistor Q.sub.1 of which the base-emitter junction and the base-collector junction are both inversely biased, photocarriers are generated and are accumulated on a capacitance associated with the base, thereby elevating the base potential from the reset level. The amount of elevation of the base potential, proportional to the amount of light, can be read out in an output line H connected to the emitter by an emitter-follower operation, by elevating the base potential at a timing t.sub.4 to establish again a forward bias state between the base and the emitter.
In the above-explained conventional method, the resetting operation is conducted in order to obtain a uniform initial potential in the transistors, prior to the start of charge accumulation at the timing t.sub.3, and the sum of the emitter currents flowing from the phototransistors into the resetting power source V.sub.VC at this operation assumes a differentiated form, as shown in FIG. 4, having a peak immediately after the start of the resetting operation. Consequently the peak current becomes larger with the increase in the number of the pixels to be simultaneously reset, leading to drawbacks such as an increased load of the external power source and a fluctuation in the power supply voltage eventually triggering a latch-up.
In order to cope with said drawbacks, the present inventors tried an increased on-resistance by varying the size of the resetting MOS transistor. This method could suppress the peak current, but resulted in another drawback of a prolonged time required for resetting operation.
The solution to the above-mentioned technical difficulties is vital for the increase in the number of pixels and for the high speed drive which are apparently the future direction of the photosensors.